The present invention relates to a two-cycle engine provided with a direct fuel injection system, and more particularly to a control system for a rotary exhaust valve provided in an exhaust passage of the engine.
In a crankcase-scavenged two-cycle engine, a scavenge port and an exhaust port are opened and closed by a piston at predetermined timings. The timings of the opening and closing of the exhaust port are symetrically positioned with respect to a bottom dead center (BDC). The opening and closing of the scavenge port are also symetrically positioned. Since the exhaust port is still opened after the scavenge port is closed, mixture escapes from the exhaust port, and hence the combustible mixture in the cylinder is not sufficiently compressed. Therefore, output torque of the engine is small and fuel consumption and exhaust gas emission are bad.
In order to solve such a problem, a uniflow scavenge system provided with a valve system has been proposed for the two-cycle engine. However, it is unrealistic to actually use such an engine, because compactness which is one of merits of the crankcase-scavenged engine is impaired.
Japanese patent application Laid-Open No. 59-25031 discloses a two-cycle engine provided with a rotary valve in an exhaust passage, where a closing timing of the valve is controlled. Japanese patent application Laid-Open No. 62-111116 discloses a system for controlling an exhaust timing control valve for opening and closing an upper portion of the exhaust port in accordance with engine speed.
Although the scavenge and exhaust timings are properly controlled so that the opening and closing timings become unsymmetrical with respect to the BDC in the above-mentioned prior arts, the torque band which is effective to drive a vehicle is narrow and the torque itself is inevitably small because of the crankcase-scavenged two-cycle engine.
More particularly, in the crankcase-scavenged two-cycle engine, derivery ratio l.sub.O, which is a ratio of intake air quantity to the displacement of the engine, is 1.0 at the most. In addition actual intake trapping efficiency, or charging efficiency of intake air in cylinders is generally lower than the estimated trapping efficiency which is calculated assuming that the air is mixed with residual gas as soon as it enters the cylinders (perfect mixing).
Although the intake ratio l.sub.O can be increased over the value of 1.0 to 1.5 by compressing intake air with a scavenge pump, the trapping efficiency decreases. On the other hand, since the optimum closing timing of the exhaust port for obtaining a maximum torque changes with an increase of engine speed, a wide torque range cannot be obtained if the exhaust port is closed at a fixed timing.